1. Field of the Invention
The present invention relates to an image scanning device and a shading compensation method thereof, and more particularly, to an image scanning device and a shading compensation method of compensating for shading distortions more precisely by generating a shading profile according to changes of ambient temperatures or characteristic changes of an image optical sensor scanning images.
2. Description of the Related Art
Image scanning devices, such as a scanner, are devices reading out in a graphic form image information represented on sheets of paper, such as pictures or drawings. Such image scanning devices are classified into a flat bed type device and a sheet feed type device.
The flat bed type device uses a mechanism in which an image optical sensor moves with respect to a document placed on a flat bed to scan the document as in a photocopier, whereas the sheet feed type device uses a mechanism in which the image optical sensor is fixed and the document moves with respect to the image optical sensor to scan documents sheet by sheet as in a facsimile machine.
Here, the image scanning device of a sheet feed type uses the image optical sensor to read out images of the document when the document is inserted between the image optical sensor and a white roller. Described in more detail, the image scanning device uses an image optical sensor to read out a to-be-scanned document (referred to as a scanning document, hereinafter) if the scanning document picked up by a transfer roller is transferred to the white roller. At this time, a contact image sensor (CIS), a charge coupled device (CCD), or the like is used for the image optical sensor.
However, the images read out by the image optical sensor, such as the CIS, the CCD, or the like, have shading distortions occurring thereon when the document is scanned, due to physical characteristics of an optical system in the image scanning device, electrical characteristics of the image optical sensor itself, contamination of the white roller, and so on. Accordingly, the image scanning device stores a shading profile in a certain memory so as to compensate for the shading distortions occurring during scanning of the document. The shading profile is a reference white color value obtained by scanning the white roller and stored as a factory default at a product adjustment stage in a factory where the image scanning devices are manufactured. That is, a conventional image scanning device compensates for the shading distortions occurring during scanning of a document to generate an output, according to the stored shading profile.
However, the image optical sensor experiences its characteristic changes due to a long-term usage or changes of an ambient temperature of the image scanning device. That is, it is preferable that the shading profile obtained at the factory adjustment stage has the characteristics as shown in an ‘A’ waveform (shown in a dotted line) of FIG. 1. However, the shading distortions occur as shown in a ‘B’ waveform of FIG. 1 due to the characteristic changes of the image scanning device.
Accordingly, if the characteristics of the image optical sensor are changed, the conventional image scanning device compensates for a scanned image according to the shading profile obtained in a state that the changes of the characteristics of the image optical sensor are not considered, thereby causing a problem that precise compensations can not be performed as compared to an actual image recorded on a document. That is, the shading profile stored in a certain memory is a compensation value to compensate for an image read out by the image optical sensor prior to its characteristic changes. Therefore, if an image read out by the characteristics-changed image optical sensor is compensated according to the stored shading profile, the shading distortions occur on the compensated image. Accordingly, the image quality of the scanned image deteriorates, thereby lowering a product reliability.